The present invention relates to a circuit for converting the differential outputs of a balanced mixed into a single-ended output. The circuit is implemented by using semiconductors without transformer matching. The circuit can be used in all balanced transistor mixers, provided that both sides of the differential output are available for use.
Because of the construction of the conventional prior-art transistor mixer, shown in FIG. 1, the balanced output is located on the collectors of the transistors and has a high impedance (approximately 100 kohm). In general, the mixer is always followed by some kind of a filter by means of which non-desirable mixing results are eliminated. The impedances of the filters, for their part, are considerably lower, approximately 2.5 kohm-50 ohm. Traditionally, the matching necessary to achieve power transfer is carried out by using transformer matching, as shown in FIG. 2. The exploitation of balancing will in this case suppress any distortion of the output signal. An alternative method of matching the output impedance of such a mixer circuit for the filters is to use resistances, as shown in FIG. 3, or to use other similar impedance matching circuits.
The output signal in the single-ended output D of the prior-art matching circuit, shown in FIG. 2, based on a transformer M, fulfills the requirements imposed on the quality of the signal i.e. a matched impedance and a low value of distortion, but in present-day receiver-transmitter applications, transformers are no longer used, since they are large in size, heavy, expensive and need to be tuned.
In most commercial mixer circuits the output impedances are matched for the filters by using corresponding resistances, such as RL, shown in FIG. 3. The problem in this case is that, in the absence of transformer circuitry, only one side, either A or B, of this differential output can be used, since the inputs of the filters are not differential.
By using the mixer without the transformer circuit, the following problems may arise at high input signal levels: the output signal is clipped asymmetrically; the output signal collapses, the mixing gain disappears; the harmonics of the output signal dominate strongly.
In the case of a high-level input signal there are thus, in both branches A and B of the differential output of the mixer of FIG. 3, mixing results which have been clipped and distorted at the peak. Such high-distortion signals of the differential outputs of a conventional mixer are shown in FIG. 4.